Electric machines have a stator and a rotor; the stator typically has a stator core with slots that house conductive stator bars. The stator bars have a straight portion housed within the slots (slot portion) blocked by wedges, and portions outside of the slots (end windings). The stator core with the stator bars may be immersed in a resin bath to impregnate all components and realize a stiff and compact structure.
In some cases a rewind (i.e. a replacement of the old stator bars with new stator bars) is needed in order to extend the lifetime of the electric machine, thus the stator bars must be removed from the stator core.
In order to remove the stator bars from the stator core, the end windings are generally cut and removed first, typically with a portable saw. Afterward the wedges are removed typically by means of a portable saw and finally the bars are stripped out from the slots.
With reference to FIG. 1, a portion of a stator core 1 with a slot 2 is shown; stator bars 3 (two in this example) are housed in the slot 2 and blocked by a wedge 4.
In order to remove the stator bars 3, an operator sits within the stator core (i.e. in its bore) and using the portable saw makes a cut 5 in the wedge 4. Thanks to this cut 5, the wedge 4 can be removed for example with the help of a hammer. Afterwards also the stator bars 3 can be removed, for example with the hammer or a different stripping device.
This way of removing the stator bars 3 from the slots 2 has some problems. In fact, cutting the wedge 4 with a portable saw is exceptionally time consuming and thus it sensibly contributes to increase the time length of the rewind. Since the rewind implies an outage of the electric machine, its time length should be reduced to the minimum.
In addition, cutting the wedge with a manually operated portable saw has the risk that during cutting the stator core is damaged. The stator core is made of a plurality of iron sheets electrically insulated from one another; this structure allows inducing a magnetic field in the whole stator core, without any circulation of eddy currents.
In case during an improper wedge cutting the stator core is damaged, in a small area close to a slot 2, a number of different iron sheets could be electrically connected each other, thus allowing a circulation of eddy currents during operation.
Even if such a circulation of eddy currents is localized, it can cause local high losses that in turn lead to local high temperatures and thus localized accelerated aging.
The localized accelerated aging can lead to machine failure, indeed stator and/or rotor bars are generally insulated with mica tapes impregnated with a resin; this resin is generally very sensitive to the temperature and requires specific operating temperatures not to be exceeded to ensure the expected lifetime of the machine.
In addition, during cutting the operator sits within the stator core; in case the stator core is relatively small it could be very troubling for the operator to sit therein and operate the portable electric saw without risks for the operator himself. In particular, it is typically very troubling cutting the wedges at the top of the stator core.
US2010/0162560 discloses a device with guides to drive the cut of the wedges; this device has the disadvantage that the operator has to hold the device in all conditions, such that working can be troubling and ineffective.